ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



199 



drawn from other populations at the same time of the year, for example by 

 Chapskii (1936) and Mansfield (1958a). 



The indicated mean crown-rump length at birth, based on 15 full-term fetuses 

 and newborn calves taken between 24 April and 26 May, is about 101 cm (range, 

 92 to 112 cm). The mean standard length at birth is about 113 cm (range, 95 to 

 123 cm). 



In each of the pregnant and postpartum females in my sample, the embryo, 

 fetus, or new placental scar was in the uterine horn adjacent to the ovary with 

 the corpus luteum. Transuterine migration of an ovum or blastocyst to the 

 opposite horn probably is not possible in the walrus because of the didelphic 

 structure of the uterus. The complete separation of the right and left uterine 

 horns affords no inter-cornual passage for the ovum or unimplanted embryo, 

 other than through the abdominal cavity or the vagina, neither of which is a 

 likely route. There are no records of ectopic, bursal, or oviductal pregnancies in 

 the walrus. 



Fetal Sex Ratio 



Of 213 embryos and fetuses, most of which were obtained in June to 

 November, 103 (48.4%) were males (Table 33). Conversely, of 395 calves, 

 ranging in age from newborn to about 3 months old, 204 (51.6%) were males. 

 Although neither of these proportions differs significantly from a 1:1 sex ratio, 

 they suggest that there is a slightly higher proportion of females than of males in 

 the fetal population and a higher proportion of males at birth. 



Table 33. Comparative numbers of male and female walrus fetuses and calves." 



Life stage, and months Males Females Total 



Fetuses 



June-August 21 23 44 



September-November 64 66 130 



December-February 6 2 8 



March-May 12 19 31 



Total 103 110 213 



Calves 



April-July 204 191 395 



^Sources: Belopol'skii (1939), Nikulin (1941), Brooks (1954), Mansfield (1958a), Loughrey 

 (1959), Krylov (1962, 1969), and J. J. Burns and F. H. Fay (unpublished data). 



Time of Birth 



The time of birth of walrus calves is generally regarded as being in May (Asdell 

 1964; Harrison 1969), but as Krylov (1969) has pointed out, there are few 

 substantive data on that point in the literature. Belopol'skii (1939) recorded one 

 "apparently recently born" calf on 14 May. Nikulin (1941) encountered no 

 females in the first half of May with a full-term fetus; he saw only calves that had 

 been born some time earlier. Brooks (1954) noted that the calves he saw in May 

 and June still retained a portion of the umbilical cord, which suggested that they 

 had been born recently. He also recorded two females with full-term fetuses on 



